feat(web): drop redundant preview pill from topbar

reaction_board/web/static/index.html

@@ -23,7 +23,6 @@
       <div class="topbar-actions">
         <a class="topbar-hint topbar-auth" href="/login" hidden>登录</a>
         <span class="topbar-links" data-topbar-links hidden></span>
-        <span class="topbar-hint topbar-preview" hidden>预览</span>
       </div>
     </header>
     <div class="preview-banner" id="preview-banner" hidden>预览数据 · 不影响真实数据库</div>
@@ -42,7 +41,6 @@
         <div class="topbar-actions">
           <a class="topbar-hint topbar-auth" href="/login" hidden>登录</a>
           <span class="topbar-links" data-topbar-links hidden></span>
-          <span class="topbar-hint topbar-preview" hidden>预览</span>
         </div>
       </div>
       <div class="nav-items" id="nav-items"></div>